Notably, BRD4 isoform switching by BRD4-ex12-SA-gRNA reduced BCL2 protein levels in MLL-X AML, but not in MLL-WT (Fig. 4i, Supplementary Fig. 10k), further highlighting that the BRD4S-to-BRD4L switch is a selective vulnerability of the former. Here, KMT2A is linked to acute myeloid leukemia.